Refer to the Invoice Templates & Styles topic on the left menu. It explains how you can create your own, personalized printed/pdf invoice template. This allows you to personalize the output as you need to. Also, you can try out other templates included with SI by selection in SI Defaults setting.
I found it the file public_html\invoices\templates\invoices\default\template.tpl and also the code:
But how can I change the format?
I was expecting something like